Sewage water admitted to municipal sewage treatment plants contains large quantities of refuse and similar waste material. The refuse and similar waste material is separated in a first dewatering step by means of a screen, whereby the dry matter contents are increased to about 9%. The refuse leaving the screen thus is moist. However, it is desirable to increase the dry matter contents further in order thereafter to normally carry the refuse away. One common manner of doing so is to make use of a device known as a piston press by means of which the dry matter contents may be increased to nearly 20%. The Swedish Patent Application 383 315 discloses a piston press of this kind This piston press consists a cylinder of considerable longitudinal dimensions, arranged in such a position that it inclines upwardly towards the outlet. A reciprocating piston is arranged in the cylinder interior and a funnel feeds refuse down into the cylinder. The piston pushes the refuse, disposed in front of the piston, towards the cylinder outlet. This piston press is characterized therein that it is shaped as an elongate pipe mounted with an inclination with respect to the horizontal plane and with its lowermost end, formed with drainage holes, positioned below the funnel, in that the stroke of the piston exceeds the length of the feed-in opening, and in that the pipe length, calculated from the feed-in opening, equals at least twice the length of the stroke. Owing to the considerable length beyond the maximum extension of the piston a braking effect acting on the refuse is created. This braking effect contributes to compacting and dewatering of the refuse. The water is drained from the waste towards the drainage apertures, owing to the inclination of the cylinder. From the piston press outlet the compacted refuse falls into a bag or similar container.
However, this piston press has certain drawbacks. The braking effect on the refuse is generated in consequence of the length of the piston press, which results in a machine of considerable longitudinal dimensions. In addition, it needs to have an upwards slope in order to function satisfactorily and in some cases this could be disadvantageous. The piston is actuated by a hydraulic cylinder by means of a hydraulic unit. This is a complex and expensive solution since it requires a multitude of components, among them an oil tank, a driving motor, pumps, and lines. Consequently, the hydraulic unit becomes comparatively bulky while at the same time it involves a definite risk for oil spillage. In addition, it is a complex operation to change the length of the stroke of the hydraulic cylinder in a hydraulic system of this kind. To be able to modify the stroke of the cylinder rapidly is often desirable in order to allow testing of changed operational parameters. Hydraulic systems do not lend themselves to such adaption in a rapid and efficient manner. Essentially, the complexity entails problems and consequential dangers of errors, high costs and limited flexibility with respect to stroke length adjustment.